Hydroformylation reactions involve the preparation of oxygenated organic compounds by the reaction of carbon monoxide and hydrogen (a.k.a., syn or synthesis gas) with carbon compounds containing olefinic unsaturation. The reaction is performed in the presence of a carbonylation catalyst and results in the formation of a compound, for example an aldehyde, which has one more carbon atom in its molecular structure than the starting olefinic feedstock. By way of example, higher alcohols may be produced in the so-called "oxo" process by hydroformylation of commercial C.sub.6 -C.sub.12 olefin fractions to an aldehyde-containing oxonation product, which on hydrogenation yields respective C.sub.7 -C.sub.13 saturated alcohols. The crude product of the hydroformylation reaction will contain catalyst, aldehydes, alcohols, unreacted feed, syn gas and by-products.
Before further processing of the crude product is possible, it is necessary to remove the catalyst therefrom. One conventional method of removing cobalt values from such a crude product is to treat the product with an alkali or acid wash technique. See U.S. Pat. No. 3,725,534 (Reisch), which issued on Apr. 3, 1973. However, this approach uses expensive raw materials and incurs problems associated with finally removing essentially all traces of cobalt from the water wash streams before being discharged.
Another conventional method involves the oxidation of the cobalt catalytic species followed by extraction as a cobalt salt in aqueous solution. See U.S. Pat. No. 2,744,921 (Mertzweiller et al.), which issued on May 8, 1956.
U.S. Pat. No. 4,625,067 (Hanin), which issued on Nov. 25, 1986, discloses still another method which involves the contacting of the crude product with a stream of stripping gas to entrain volatile cobalt compounds, characterized in that the contacting is performed in the presence of water and aqueous acid to dissolve those cobalt values not entrained in the gas under the conditions of temperature and pressure employed for the contacting, and the aqueous phase is subsequently separated from the organic hydroformylation reaction product.
Although the stripping method disclosed in the Hanin patent overcomes the disposal and chemical additive costs of the caustic/acidification method of Reisch, it has the disadvantage that when lower carbon number olefins (e.g., C.sub.7 and below) are used as the feedstock, unreacted compounds such as olefins and/or paraffins are stripped out together with the volatile cobalt compounds. These olefins and/or paraffins are then absorbed into the olefinic feedstock and recycled to the oxo reactor. This occurs because lower carbon number feedstocks such as heptene have roughly the same volatility as the cobalt specie, thereby causing it to be entrained together with the volatile cobalt and taken out overhead. Light hydrocarbons which are absorbed into the olefinic feedstock rapidly build up within the cobalt recovery system causing an undesirable decrease in net olefin feed rate.
The present inventor has developed a method of recovering cobalt values which does not cause the build up of unreacted light hydrocarbons within the system, thereby avoiding a decrease in the olefin feed rate. This is accomplished by providing a demetalling step prior to the stripping step which produces a substantially cobalt-free organic hydroformylation reaction product and water soluble cobaltous salt aqueous product. The organic hydroformylation reaction product is diverted for further downstream treatment, while the water soluble cobaltous salt aqueous product is concentrated, converted to cobalt carbonyl and stripped of volatile cobalts substantially free of any light hydrocarbons.
With the addition of the acid-air demetalling step and stream re-routing the same equipment can be used for treating a wide range of carbon number feedstocks, reduce chemical consumption and produce lower quantities of wastewater than conventional systems. Moreover, the method according to the present invention is particularly suited for a closed system, i.e., substantially reduced organic acid and alcohol stream make-up is required.
The present invention also provides many additional advantages which shall become apparent as described below.